The present invention relates to a heterogeneous electrochemical supercapacitor (HES), and to a method for manufacturing such a capacitor. More particularly, the present invention is directed to an improved HES. A HES of the present invention exhibits superior operating parameters in comparison to other electrochemical capacitors of known design.
There is an increasing focus on the use of double electric layer (DEL) electrochemical capacitors as a means for storing electrical energy. Among known electrochemical capacitors the HES typically exhibits the highest specific energy while simultaneously providing for electrical energy storage at the lowest cost. In the electrodes of conventional electrochemical capacitors, electric charge exists in a free state and the energy of both electrodes is potential energy. Unlike conventional capacitors, charge carriers appear in the non-polarizable electrode of a HES due to phase transition of the second kind, and exist in the polarizable electrode in a free or loosely coupled state. Since the energy associated with the polarizable electrode is potential energy and energy of the non-polarizable electrode is chemical energy, the nature of origin of electric charge and energy with respect to the electrodes differs and, hence, the proposed supercapacitor is considered to be heterogeneous. Such supercapacitors can efficiently store and redistribute a large amount of electrical energy. For purposes of illustration, and not limitation, such capacitors may be used: as a main power supply; as a back-up power supply; for power quality assurance (i.e., to compensate for short-term power “surges”, “spikes”, and “skips” common to a utility-supplied source of electrical power); to provide load-leveling by storing an amount of electrical energy provided during off-peak hours and re-distributing said electrical energy during periods of peak demand; and as a primary or secondary power source for a variety of vehicles.
A HES typically uses only lead and activated carbon as the primary components for manufacturing its electrodes. A HES is typically of double electric layer (DEL) design. A DEL capacitor typically employs a pair of electrodes that are arranged in a spaced apart relationship, and between which resides an electrolyte. The electrolyte is generally aqueous in nature. A separator typically also resides in the space between the electrodes. One or both of the electrodes may store electrical energy through a double layer electrochemical mechanism. In a double electric layer storage process, a layer of electrons forms at the electrode side of the electrode/electrolyte interface. A layer of positive ions also forms on the electrolyte side of the electrode/electrolyte interface. The voltage across the interface between the electrode and electrolyte increases with charge accumulation, and is eventually released during discharge of the capacitor.
One or both of the electrodes of a DEL capacitor may generally be polarizable electrodes. A polarizable electrode may comprise, for example, an active material and a current collector to which the active material is affixed. The most commonly employed active material is likely one of a plurality of activated carbon materials. Activated carbon materials are inexpensive and have a high specific surface are per unit mass. Negative electrodes are typically formed from activated carbon materials in the form of an activated carbon powder and binder, or from woven or non-woven activated carbon fiber materials. However, preparation of DEL electrodes from an activated carbon powder is often preferable due to its lower cost. Positive electrodes may be formed from various conductive materials, particularly metals.
As stated above, in a typical DEL capacitor, one or both of the electrodes may be polarizable. However, constructing a DEL capacitor with one polarizable electrode and one non-polarizable electrode has been shown to provide the DEL capacitor with a specific energy capacity that is greater than that of a capacitor with two polarizable electrodes. In such a DEL capacitor, charge storage at the non-polarizable electrode occurs as a result of oxidation and reduction reactions at the interface of the non-polarizable electrode and the electrolyte. Such an electrode is commonly said to exhibit Faradic pseudocapacitive behavior. In a HES of DEL design, the nonpolarizable electrode is typically comprised substantially of lead.
At least the negative electrode of such a DEL capacitor is typically affixed by some means to a current collector. Current collectors are commonly constructed of a material that exhibits electrical conductivity—typically a metal. As at least a portion of the current collector, along with the electrode material, must reside in the electrolyte, it is preferable that the collector material will not react adversely thereto. For example, the electrolyte of a DEL capacitor may consist of an aqueous sulfuric acid. In such a case, certain precautions such as, for example, coating or otherwise protecting the portion of the current collector exposed to the electrolyte may need to be undertaken, as the sulfuric acid electrolyte may erode, or otherwise degrade the current collector material.
Various embodiments of electrochemical capacitors are currently known. However, there are disadvantages to many of these known electrochemical capacitor designs. For example, one concern with the use of electrochemical capacitors is cost—both the cost to manufacture the capacitors, and the cost of storing energy therewith. With the exception of a HES, known electrochemical capacitors generally employ materials such as aluminum, nickel, niobium, ruthenium, tantalum, titanium, and tungsten in their construction. These materials are considerably more expensive than the lead material typically used in a HES. Consequently, both the cost to manufacture and the cost to store energy using electrochemical capacitors of typical design can often be prohibitive.